The group includes moderates and senators known for crossing party lines, a few who are retiring and some who had already broken with their party to support same-sex marriage.
The second-term Democrat was the first openly gay woman elected to Congress. Now she is leading the effort to rally Republican support for legislation that would protect marriage equality nationwide.
The larger-than-expected G.O.P. vote in the House in support of legislation to codify marriage equality caught both parties off guard, suggesting there could be a narrow path to enactment.